1. Field of the Invention
The present invention relates to a camera. More particularly, this invention is concerned with a film transport mechanism for a camera which rotates forwardly or reversely a spool lying in a film cassette so as to feed film wound about the spool from the film cassette or to wind up or rewind the film.
2. Description of the Related Art
In a conventional camera in which a film cassette containing film up to a leader is mounted, a spool lying in the film cassette is rotated forwardly or reversely by means of a drive motor so that film wound about the spool is fed from the film cassette, or wound up or rewound.
In the past, various proposals have been made for a film transport mechanism for such a camera. For example, a film transport unit for a camera disclosed in Japanese Patent Laid-Open No. 3-287150 (1991) is a mechanism including a drive shaft coupled with one end of a spool in a film cassette and designed to drive and rotate the spool by means of an electric motor, and two systems of transmission routes that are provided by a planetary gear train. A rotational driving force transmitted from the electric motor via a rotation transmission system is directed to one of the two systems of transmission routes leading to the drive shaft projecting to the film cassette which is selected by a switching and transmitting means whose switching is controlled according to the direction of rotation of the electric motor. When driven and rotated by means of the driving force, the spool feeds film from the film cassette, winds it up, or rewinds it into the film cassette.
The film transport unit disclosed in the Japanese Patent Laid-Open No. 3-287150 has two planetary clutches attached to an L-shared lever. Thus, the mechanism is complex, and a large planar space is required as an area for movements. Besides a film cassette chamber occupies a large space. For realizing a compact camera, therefore, spaces above and below the film cassette chamber in a camera must be designed as small as possible. However, the complexity of the mechanism imposes limitations on the compactness.
A film transport unit for a camera disclosed in Japanese Patent Laid-Open No. 2-67535 (1990) has a driving force transmission route switching means made up of two one-way clutches and a planetary gear train. Herein, routes of transmitting a driving force to a spool in a cassette are switched according to a direction of rotation made by a drive motor, whereby film is fed, wound up, or rewound. For effecting the switching, a reduction ratio in film rewind speed is made larger than a reduction ratio in film take-up speed. This mechanism prevents the one-way clutches from transmitting the rotational motion enabling film rewind to a film take-up spool. A speed reduction ratio in a gear train for feeding film is made larger than the one in a gear train for taking up film. When film is fed and then wound about a take-up spool, since the take-up spool rotates at a higher speed, the one-way clutches that have been transmitting a driving force enabling film feed are freed.
In the film transport unit disclosed in the Japanese Patent Laid-Open No. 2-67535, however, the magnitudes of speed reduction ratios set in reduction gears restrain innovative designing. Even if an attempt is made to realize a small-sized mechanism by equalizing the speed reduction ratio in a film rewind with the one in a film feed and thus reducing the number of gears, the attempt fails.
An electric film rewind for a camera disclosed in Japanese Examined Utility Model Publication No. 56-30813 includes a clutch mechanism in which a helical gear and a constraining spring are used to allow an axis having the habit of turning in one direction to change directions of rotation made by a drive motor. Thus, gears are disengaged from each other and a member for transmitting a driving force enabling film rewind is clutched.
In the film transport unit disclosed in the Japanese Examined Utility Model Publication No. 56-30813, gears engage with each other as efficiently as with a small backlash ranging from 0.02 to 0.05 mm between them and transmit a driving force. When the clutch mechanism works, if the gears go up, one gear's timing of engaging with a mate may be inconsistent with the other's timing. In this case, the gears are struck off each other. Thus, the gears may not engage with each other very well.